Spectrometric analysis entails the precise measurement of the interaction between a sample (analyte) and an energy source in order to determine the chemical composition of the aforementioned analyte. Techniques of spectrometric analysis vary both in the state in which an analyte is placed prior to testing, and in the type of energy to which the analyte is exposed. However, all spectrometric techniques are based upon relating the energy-dependent behavior of an analyte to its constituent quantity and quality.
In emission spectrometry the analyte to be tested is supplied with energy from a non-radiative external energy source, usually heat from a plasma flame or electric wire. Upon exposure to an external energy source, the analyte gains energy, and typically re-emits this energy in the form of photons. The quantity and scatter distribution of these released photons is then measured by a light sensitive spectrometer, and used for quantitation, since the energy emission pattern of an analyte is specific for each constituent of that analyte. Thus, allowing a quantitative analysis of the elemental composition of that analyte to be made.
The Inductively Coupled Argon Plasma-Optical Emission Spectrometer (ICP-OES) is a species of plasma spectrometer that can quantitatively analyze various sample/analyte types to determine their elemental composition. Common sample sources include: water, plant and animal tissues, geological specimens and industrial samples. Plasma spectrometers use a radio frequency and a stream of argon in an open-ended quartz tube to generate plasma whose temperature can reach 10,000 degrees centigrade. The hot plasma created in this way is flame-like in appearance and is as hot as the surface of the sun. A stream of argon then carries an aerosol of the sample to be analyzed into the central channel of the plasma. As the sample encounters the hotter portion of the plasma its atoms go from their ground state to an excited state or, become ionizable, a situation in which some of the sample's electrons are stripped from outer valence shells. Eventually the electrons return to their ground states and during this change in energy status, they release a characteristic wavelength of light for each element present in the sample. It is this characteristic or signature spectra pattern of light, which is used to identify given elements.
In a conventional radially-viewed ICP-OES, the emitted light is viewed from the side of a vertically oriented plasma and focused on the entrance slit of the spectrometer. The spectrometer separates the emitted light by wavelength and measures the intensity of the generated light. In an axially-viewed ICP-OES, the plasma is tilted 90.degree. degrees on its side, and the optical interface images the central channel of the plasma onto the entrance slit of the spectrometer for viewing. It is the central channel of the plasma that emits the light with the best signal-to-noise ratio, resulting in a ten to thirty fold improvement in sensitivity of measurement over its radially viewed counterpart.
However, current designs of axially viewed ICP-OES are prone to matrix interference because the entrance optics are designed with a long depth of field. As a result the entire axial channel of the plasma, including its tip and base is imaged onto the entrance slit of the spectrometer through the use of long focal length lenses or mirrors. This generally causes a 10-15% loss of recovery of some elements in a 1000 ppm (parts per million) calcium matrix. In addition, the cost of a new axially viewed spectrometer is considerably higher than converting a radially-viewed spectrometer to an axially-viewed spectrometer. This economic factor often limits who can purchase and use axially-viewed spectrometers. The invention disclosed herein is designed to alleviate the aforementioned problems by providing a device that can be integrated into a new spectrometer entirely or the optical interface can comprise an apparatus with the capability to be reversibly attached to an existing spectrometer. In either case the optical configuration disclosed herein will reduce the matrix effects that occur in both radial and axially-viewed spectrometers.